In a cellular wireless communication system, a channel status is deteriorated due to a geographical factor inside a cell, a distance between User Equipment (UE) and a base station, or movement of the UE, so that communication between the UE and the base station is not promptly performed. More particularly, when the UE is located inside a closed building such as an office or a house, a channel between a base station and the UE is deteriorated, because a shadow region is formed. The UE located in the shadow region cannot promptly perform communication with the base station.
Recently, a femto cell concept is proposed in order to provide a high level data service to more users while solving a service limitation of the indoor shadow region. The femto cell is a small cell area formed by a femto base station (femto eNB or home dNB) which accesses a mobile communication core network via a broadband network installed inside an indoor space. Since the femto cell has a very small cell area compared to a macro cell, a plurality of femto cells may be installed inside one macro cell area.
A macro base station may have interferences from a plurality of femto cells existing inside a cell area of the macro base station. Therefore, to reduce an amount of interference by a femto base station to a macro base station, a conventional art suggests a technique of turning off transmitter power of the femto base station which does not currently provide a service to the UE.
The technique of turning on/off transmitter power of the femto base station is described with reference to FIGS. 1A to 1B. First, when a UE 100 which receives a service from a macro base station 102 as illustrated in FIG. 1A moves and accesses a specific femto base station 104 as illustrated in FIG. 1B, since the femto base station 104 turns off the power of its transmitter but turns on the power of its receiver, the femto base station 104 receives an uplink signal from the UE 100 and measures (step 110) power thereof. At this point, when the power of the measured uplink reception signal has an intensity of a threshold or more, the femto base station 104 turns on the power of the transmitter and transmits control channels to the UE 100 as illustrated in FIG. 1C, and the UE 100 measures the control channel of the femto base station 104, and reports (step 130) measurement results to the macro base station 102 as illustrated in FIG. 1D. The macro base station 102 determines whether to allow a handover of the UE 100. When the handover is required, the macro base station 102 transmits (steps 140 and 142) a handover message to a relevant femto base station 104. At this point, the macro base station 102 identifies the relevant femto base station using a Physical Cell IDentifier (PCID) received from the UE 100. There may exist a plurality of femto base stations having the same PCID inside the cell area of the macro base station 102. Therefore, a handover message of the macro base station 102 is transmitted to a different femto base station 106 having the same PCID as that of the femto base station 104 as illustrated in FIG. 1E.
On the other hand, when the handover of the UE is not required, the macro base station 102 does not take a special measure for the femto base station 104. Therefore, the femto base station 104 stands by for a predetermined time until a timer expires, and turns off (160) the power of the transmitter as illustrated in FIG. 1F. That is, the femto base station 104 turns on the power of the transmitter and operates the timer, and when an event for a handover does not occur for the predetermined time, turns off the power of the transmitter.
As described above, according to the conventional art, a femto base station manages the power on/off of the transmitter by operating the timer. Therefore, even when a handover to the femto base station does not occur, the femto base station stands by with the power of its transmitter turned on until the timer expires, so that an interference may be generated to a neighbor macro base station.
Also, since the UE reports only a PCID to the macro base station while reporting the measurement results, the macro base station cannot exactly recognize a femto base station to which the UE is to perform a handover and thus transmits handover messages to even the other femto base stations except the relevant femto base station to which the UE is to perform the handover. Accordingly, the power of transmitters of unnecessary femto base stations is turned on.
The above-described conventional technique of controlling the transmitter power of a femto base station unnecessarily turns on the transmitter power, and requires a long time until the transmitter power is turned off, so that a large amount of interference may be still generated to a macro base station.